System and method for adaptive RF ablation

ABSTRACT

A medical method, device, and system are provided, including advancing an ablation element of a medical device into contact with tissue to be treated, selecting a power level of energy to ablate the tissue, delivering energy at the selected power level to the ablation element, determining whether the ablation element is in continuous contact with the tissue, and reducing the selected power level when the ablation element ceases to be in continuous contact with the tissue.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional Application No.61/417,629 filed on Nov. 29, 2010.

Cross-reference is hereby made to the commonly-assigned related U.S.application Ser. No. 13/096,255, entitled “System and Method forAdaptive RF Ablation” filed concurrently herewith and incorporatedherein by reference in it's entirety.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

N/A

FIELD OF THE INVENTION

The present invention relates to medical systems and methods forablation of tissue, including cardiac tissue.

BACKGROUND

Medical procedures are used to treat a variety of cardiovasculardefects, such as cardiac arrhythmias, atrial fibrillation, and otherirregularities in the transmission of electrical impulses through theheart. Such medical procedures may involve ablation of the specifictissue that cause or transmit the irregular electrical impulses, e.g.,creating lesions or other anatomical effects that disrupt or blockelectrical pathways through the targeted tissue so as to allow the othertissues to function properly. In the treatment of cardiac arrhythmiasfor example, a specific area of cardiac tissue having aberrantelectrical activity (e.g. focal trigger, slow conduction, excessivelyrapid repolarization, fractionated electrogram, etc.) may be identifiedfirst and then treated.

One example of a type of ablation system involves the delivery ofradiofrequency (“RF”) energy to the tissue selected for treatment. RFablation systems may include a power source or RF generator, and one ormore medical devices having at least one ablation element or electrodecoupled to the power source. The medical device may be a flexiblecatheter having a handle at a proximal end and an ablation electrodenear a distal end, or may have an array of electrodes which may beconfigured on one or more carrier arms. Examples of an RF generator andmedical ablation catheters having various configurations are illustratedin FIGS. 1-7. One or more sensors may also be provided, such as atemperature sensor, thermocouple, or a sensor for another parameter(such as contact assessment, pressure, etc.), which may be arranged ator near the ablation electrodes. The sensors may be placed near to oneor more of the ablating surface of each electrode, or at the interfacebetween the electrode and the tissue to be treated. Such a system mayalso include one or more external electrodes touching the skin of thepatient, which may be called “indifferent” electrodes, also coupled tothe power source. After mapping and diagnosing the electricalirregularities, a physician may decide to treat the patient by ablatingcardiac tissue. FIG. 8 shows a stylized depiction of an ablation systemin use during a medical treatment of the heart of a patient.

It is desirable to enable and ensure continuous contact during anablation procedure between each ablation element or electrode and thecorresponding selected tissue. It is also desirable to maintain aconstant electrode temperature during the ablation, at a valuesufficiently high to ensure that lesions are created, but not so highthat there is a risk of charring and coagulum formation. Feedbackcontrollers that are responsive to a temperature measured at or near theelectrodes may be employed to maintain electrode temperature. Sometimesthe tissue selected for treatment may be moving, such as for examplecardiac tissue of a beating heart, or during the movements associatedwith respiration.

During such movement, one or more ablation elements may lose contact orbe in only intermittent contact with the tissue. When tissue contact islost, the temperature of the ablation element will ordinarily decrease.In response, it is possible that a feedback controller of an ablationsystem may temporarily increase power output from the power source. Sucha response may be an undesirable reaction to the temperature feedbacksignal, since the decreased temperature is caused by the loss of firmand continuous tissue contact, and not due to a change in the ablationconditions that would require additional power supplied to the ablationelements.

To provide more effective, safe and efficient medical treatments, it isdesirable to optimize the ablation system and method of use to avoidexcessive local heat which may cause the formation of coagulum. It isalso desirable to monitor and recognize the level and character ofcontact by an ablation element with the corresponding tissue to betreated, and respond accordingly.

SUMMARY OF THE INVENTION

The present invention advantageously provides a medical device, system,and method for treating a patient by delivering energy to ablate tissue.The energy may be reduced during periods when an ablation element is notin contact, or has intermittent contact, with the tissue. In particular,a medical method is provided, including advancing an ablation element ofa medical device into contact with tissue to be treated, selecting apower level of energy to ablate the tissue, delivering energy at theselected power level to the ablation element, determining whether theablation element is in continuous contact with the tissue, and reducingor maintaining the selected power level when the ablation element ceasesto be in continuous contact with the tissue. In a particular example,power may be provided to one or more ablation elements or electrodesuntil the electrode reaches a target temperature. The subsequent powerdelivery is then limited to the power delivery characteristics (e.g.,such as duty cycle) that resulted in reaching the target temperature. Inother words, the delivered power characteristics that resulted in theattained target temperature are set as a threshold for subsequent powerdelivery during the treatment. If the temperature of the electrode laterdrops below the previously-attained target temperature under the same orsubstantially similar power delivery conditions, an alert may begenerated indicating that the electrode has lost sufficient contact withthe target tissue.

A medical system is also provided, including a medical device having anablation electrode, and a source of RF energy coupled to the ablationelement, the source having a duty cycle, and having a variable poweroutput, wherein the source of RF energy has a duty cycle with a baseperiod between approximately 5 ms and approximately 20 ms.

A medical system is also provided, including a medical device having anablation electrode and a temperature sensor, a source of RF energy inelectrical communication with the ablation element, the source having avariable power output with a duty cycle of selectable duration within abase frequency, and a proportional-integral-derivative controllercoupled to the temperature sensor and the source of RF energy, whereinthe integral portion of the controller has a period at least equal tothe duration of a heartbeat.

A medical method is also provided, including advancing an electrode of amedical device into contact with tissue to be treated, monitoring atemperature of the electrode, selecting a desired temperature and athreshold variation, delivering energy to the electrode, calculating anaverage temperature of the electrode, calculating a difference betweenthe temperature and the average temperature, calculating a continuityvalue by subtracting the difference from the desired temperature, andreducing the desired temperature when the continuity value exceeds thethreshold variation.

A medical method is also provided, including advancing an electrode of amedical device into contact with tissue to be treated, monitoring atemperature of the electrode, selecting a desired temperature and athreshold value, delivering energy at a duty cycle value associated withthe temperature threshold to the electrode, setting a duty cycle limitequal to an initial duty cycle value, limiting the energy to a maximumof the duty cycle limit when the temperature value exceeds thethreshold.

A medical method is also provided, including advancing an electrode of amedical device into contact with tissue to be treated, selecting adesired maximum power, delivering energy from a power source at a dutycycle value to the electrode, monitoring power produced by the powersource, calculating average power produced by the power source, limitingthe average power to the desired maximum power, setting a maximum dutycycle equal to the current duty cycle value when the power is at leastequal to the desired maximum power, and thereafter limiting the dutycycle value to the maximum duty cycle.

BRIEF DESCRIPTION OF THE DRAWINGS

A more complete understanding of the present invention, and theattendant advantages and features thereof, will be more readilyunderstood by reference to the following detailed description whenconsidered in conjunction with the accompanying drawings, wherein:

FIG. 1 is an illustration of an exemplary medical radiofrequency signalgenerator constructed in accordance with the principles of the presentinvention;

FIG. 2 is an illustration of an exemplary medical device constructed inaccordance with the principles of the present invention;

FIG. 3 is an illustration of another exemplary medical deviceconstructed in accordance with the principles of the present invention;

FIG. 4 is an illustration of a portion of the medical device of FIG. 3;

FIG. 5 is an illustration of a portion of the medical device of FIG. 3;

FIG. 6 is an illustration of another exemplary medical deviceconstructed in accordance with the principles of the present invention;

FIG. 7 is an illustration of yet another exemplary medical deviceconstructed in accordance with the principles of the present invention;

FIG. 8 is an illustration of an exemplary medical device during amedical procedure;

FIG. 9 is a flow chart of a medical method of use of the medical devicesof FIGS. 1-8 in accordance with the principles of the present invention;

FIG. 9A is an illustration of an exemplary duty cycle of a power source;

FIG. 10 is another flow chart of a medical method of use of the medicalsystems of FIGS. 1-8 in accordance with the principles of the presentinvention;

FIG. 11 is a graph illustrating exemplary parameters during the medicalmethod of FIG. 10;

FIG. 12 is a graph illustrating exemplary parameters during the medicalmethod of FIG. 10;

FIG. 13 is a graph illustrating exemplary parameters during the medicalmethod of FIG. 10;

FIG. 14 is another flow chart of a medical method of use of the medicalsystems of FIGS. 1-8 in accordance with the principles of the presentinvention;

FIG. 15 is a graph illustrating exemplary parameters during the medicalmethod of FIG. 14;

FIG. 16 is a graph illustrating exemplary parameters during the medicalmethod of FIG. 14;

FIG. 17 is a graph illustrating exemplary parameters during the medicalmethod of FIG. 14; and

FIG. 18 is a graph illustrating exemplary parameters during the medicalmethod of FIG. 14.

DETAILED DESCRIPTION OF THE INVENTION

The present invention provides medical devices, systems and methods ofuse thereof for treating a patient, which may include ablating one ormore selected tissue regions and providing a feedback or monitoringmechanism for determining whether an ablation device or element is incontinuous contact with the selected tissue, and modifying the operationof the device accordingly. Referring now to the drawings in which likereference designators refer to like elements, there is shown in FIG. 1an exemplary embodiment of a power source such as for example an RFgenerator constructed in accordance with the principles of the presentinvention, designated generally as 10. Of note, the device componentshave been represented where appropriate by conventional symbols in thedrawings, showing only those specific details that are pertinent tounderstanding the embodiments of the present invention so as not toobscure the disclosure with details that will be readily apparent tothose of ordinary skill in the art having the benefit of the descriptionherein. Moreover, while certain embodiments or figures described hereinmay illustrate features not expressly indicated on other figures orembodiments, it is understood that the features and components of thesystem and devices disclosed herein may be included in a variety ofdifferent combinations or configurations without departing from thescope and spirit of the invention.

As shown in FIG. 1, the power source 10 may generally include a displayor monitor, a console, operating controls, and couplings for connectionto one or more medical devices, one or more patient return or“indifferent” electrodes, an ECG, a power cable, and/or other operatingequipment. The power source 10 may have electronic circuitry to producethe desired ablation energy, to deliver it to the ablation elements of amedical device, to obtain feedback information or parameters from othersensors, and to operate, adjust, modulate or cease providing theablation energy during a medical treatment of a patient, as well as todisplay or otherwise inform the physician.

Generally, the power source 10 may be operated in various modes whichmay be selected by the physician. For example, ablation energy may besupplied to one or more ablation elements in a bipolar mode, a unipolarmode, or a combination bipolar and unipolar mode. A unipolar mode ofoperation involves delivering energy between one or more ablationelements on a medical device and one or more patient return elementstouching the skin of the patient. A bipolar mode of operation involvesdelivering energy between at least two ablation elements on a medicaldevice. And a combination mode of operation involves delivering energyin both bipolar and unipolar modes simultaneously and/or intermittently.When in a combination mode of operation, it may be possible to selectvarious ratios of activity or ablation energy among the bipolar andunipolar modes, including for example ratios such as 1:1, 2:1, or 4:1(bipolar:unipolar).

The medical devices coupled to the power source 10 may be catheters orsurgical probes, including for example an electrophysiology catheterhaving diagnostic and/or treatment components positionable at or near atarget tissue region. For example, the medical device 12 illustrated inFIG. 2 may have a shape and dimensions to reach various treatmentssites, such as intraluminal access to vascular anatomy, including forexample transseptal access to the left atrium of a patient's heart forsubsequent treatment or ablation. The medical device 12 may generallydefine an elongated, flexible catheter body 14 having a distal treatmentassembly 16, as well as a handle assembly 18 at or near a proximal endof the catheter body. The distal treatment assembly 16 may, for example,include one or more ablation elements such as electrodes 20, each ofwhich may be electrically coupled to the power source 10. The distaltreatment assembly 16 of the medical device 12 has a linear shape, witha plurality of ablation elements or electrodes 20. The shaft may be bothflexible and resilient, with sufficient column strength facilitatingsteady contact with tissue. This improves signal fidelity in diagnosingcontacted tissue as well as improve therapeutic thermal exchange betweenthe device and contacted tissue. The proximal handle assembly 18 has arotational actuator 22 for manipulating, bending, steering and/orreshaping the distal treatment assembly into various desired shapes,curves, etc.

FIGS. 3-5 show a medical device or ablation catheter 24 with a cathetershaft and a distal treatment assembly 26 with compound carrier armswhich may be resilient, so that in a deployed configuration theelectrodes 28 have a generally planar arrangement. Similar to themedical device of FIG. 2, the distal treatment assembly 26 may be usedfor bipolar ablation, monopolar ablation, or a combination thereof. Aproximal handle 30 has a rotational actuator 32 for manipulating adistal portion of the ablation catheter, and a linear actuator 34. Thelinear actuator 32 can advance the distal treatment assembly 26 distallybeyond the catheter shaft, and retract the distal treatment assembly 26proximally inside the catheter shaft. When the distal treatment assembly26 is advanced distally, it may resiliently expand from a compressedarrangement inside the catheter shaft to the deployed arrangement shownin FIGS. 4 and 5.

A distal treatment assembly portion of a medical device or catheter 36shown in FIG. 6 has a resilient framework of carrier arms 38, in whichthe electrodes 40 have a proximally-directed configuration, which mayfor example be used for transseptal treatments of a patient's heart.Another distal treatment assembly portion of a medical device orcatheter 42 is depicted in FIG. 7, which has a distal treatment assemblyhaving a deployed configuration in which the electrodes 44 have anadjustable linear, planar, or spiral configuration.

An indifferent or patient return electrode 46 may also be provided, asshown in FIG. 8 during an exemplary treatment of a patient's heart. Thepatient return electrode 46 may include a conductive pad having agreater surface area than the electrodes. The patient return electrodemay be external to the patient, for example in contact with thepatient's skin through an adhesive attachment to the back of thepatient, and may be operably coupled to an ECG interface unit and/ordirectly to the power source or RF generator.

Accordingly, medical systems and devices may be used to investigate andtreat aberrant electrical impulses or signals in a selected tissueregion, such as for example in the cardiac tissues of a patient's heart.A distal treatment assembly of a medical device may be advanced throughthe patient's vasculature via the femoral artery or other access routeand along a previously inserted guidewire. The distal treatment assemblymay then be advanced, for example, into the right atrium and intoproximity of a pulmonary vein.

Power sources such as RF generators may produce power according to aduty cycle, an example of which is shown in FIG. 9A. A duty cycle isperiodic, and may be calculated as the fraction of time that a system(such as a power source) is in an active state, as opposed to aninactive state. For example, the duty cycle may equal a time that thepower source is active divided by the period of the function oroperation of the power source. The duty cycle may vary or be adjustedmany times during the course of a single procedure, based on temperatureand impedance feedback from the site or local area of the ablationelement or array. Other means of controlling the power output arepossible as well, including varying the voltage output of the RFgenerator.

Sensors on the medical device may provide feedback to the system whichcan be used to control the power source and provide a safe and effectiveablation. In other words, the ablation system may continuously monitorthe power source and the local conditions near each ablation element,modulating operation of the power source accordingly. One example of acontrol system is a proportional-integral-derivative (“PID”) controller,which is a control algorithm using a feedback loop that calculates adifference between the process variable, i.e., the current conditions,properties, or feedback, and a desired goal value or setpoint. Thisdifference may be called a gap value or error signal. The controllerthen adjusts the power source operation, which may include the dutycycle, to minimize the value of the error signal or gap value betweenthe process variable and the setpoint. The PID controller may use asoftware program or algorithm to evaluate three separate parameters, theproportional, integral, and derivative values. The proportional value isbased on the current error signal times a proportional gain, theintegral value is based on the sum of recent error signals times anintegral gain, and the derivative value is based on the rate at whicherror signal has been changing, times a derivative gain. The controlleroutput is the sum of the proportional, integral, and derivative values.In addition, the integral term of the controller has a parameter, calledthe integral period, that determines the length of time for which thepast error signals are summed or integrated. An integral period ofsufficient duration allows the controller to act as a low-pass filter,rejecting high-frequency signals that would otherwise cause undesirablevariations in the output of the controller. If the integral andderivative gain values are both zero, the controller's output dependsonly on the proportional gain times the error signal. This is calledproportional or P-control. If the derivative gain is zero, and theproportional and integral gains are non-zero, then the controller iscalled a P-I controller.

In the case of a medical method in which the power source is an RFgenerator, and in which a medical device has an ablation electrode, apower level may be selected including the selection of a desired maximumpower. The power source may deliver energy at variable levels controlledby using a duty cycle as shown in FIG. 9A. The source of energy coupledto the ablation element thus has a duty cycle of selectable durationwithin a base frequency. In other words, the duty cycle is the fractionof T, the time when the power source is active, divided by T, the periodof the duty cycle. This variable duty cycle produces a selectable orvariable power output. To avoid generation of coagulum during anablation procedure, it may be suitable to increase the fidelity of thepower control system by reducing the wavelength or base frequency. Sucha lower base frequency may result in better control of the power source,and may improve heat dissipation from the ablation element. The shorterduty cycle base frequency means each instance in which an RF ablationsystem generates energy and associated heat has a shorter active period.Such a shorter active period allows heat to dissipate more effectivelythrough conduction, convention, or fluid flow including a liquid such asblood flow. A corresponding increase in thermal dissipation reduces apossible opportunity for coagulum to form. In a specific example, an RFgenerator may generally have a duty cycle with a base period ofapproximately 15-20 ms, with a particular example being 17.6 ms, whichmay be reduced to approximately half of that amount at 7.5-10 ms, with aparticular example being 8.8 ms. In other words, the period of the dutycycle may be reduced to a time of at most 10 ms, for example. Such ashorter duty cycle period results in shorter activation times, whichprovides more efficient heat dissipation, as well as greater fidelity incontrol and responsiveness during operation of the RF generator.

A medical system may include a medical device having an ablation elementand a feedback sensor, a source of energy having variable power output,and a proportional-integral-derivative (“PID”) controller. The energysource is operatively coupled with the ablation element and feedbacksensor. The PID controller is coupled to the feedback sensor and thesource of energy, and the parameters of the PID controller may beselected to reduce a possibility of coagulum.

A medical system may have a medical device with an ablation element anda feedback sensor, a source of energy having a variable power outputwhich is in operative communication with the ablation element andfeedback sensor, and a proportional-integral-derivative (“PID”)controller coupled to the feedback sensor and the source of energy, theparameters of the PID controller may be selected to reduce a possibilityof coagulum. A goal may be set for operation of the ablation element,which may for example be a selected temperature, and which is adjustedover time during the course of a medical treatment. The feedback sensorprovides information as to current or instantaneous conditions at ornear the ablation element. If there is a difference between the goal ordesired conditions and the actual conditions observed by the feedbacksensor, the controller attempts to minimize this difference.Accordingly, the proportional, integral, and derivative parameters maybe independently adjusted or tuned for performance, accuracy, andresponsiveness. For example, given a current difference between desiredand actual conditions, and a series of difference observations overtime, the proportional parameter may correspond to the present orinstantaneous difference, the integral parameter may correspond to anaggregate of past differences observed over time, and the derivativeparameter may correspond to a prediction of future differences based ona current rate of change in the difference between desired and observedvalues. A weighted combination of these three parameters may be used toadjust the power source, during the course of an ablation medicaltreatment.

In a particular example of an ablation system for treatment of cardiactissues, the integral parameter of a PID controller may be selected tohave a longer period, for example at least equal to the duration of aheartbeat. Such a longer period for the integration parameter of a PIDcontroller may slow down the response of controller to momentaryoscillations, and may reduce or avoid tracking or chasing of the powersource to fluctuations in feedback observations of local conditions.Accordingly, local conditions at an interface between the ablationelement and the tissue to be treated may experience fewer suddenchanges, and of lesser magnitude. In the case of a temperature sensor, alonger integral period may avoid temperature oscillations caused bymotion, such as the movement of cardiac tissue during a heartbeat.

In an exemplary use of a medical system as illustrated in the flowdiagram of FIG. 9, the medical system is first prepared for ablation,and the ablation system is set up. One or more ablation elements areplaced in contact with tissue to be treated. Various ablation parametersare determined, which may include for example the intended duration ofablation, desired power, and/or desired temperature. A power level ofenergy to ablate tissue is selected, and energy is delivered at theselected power level to the ablation element. The medical systemcontinuously monitors feedback information from the ablation element andevaluates whether the ablation element is in continuous contact with thetissue, using one or more of the techniques described below. If theablation element is not in continuous contact with the tissue, the powerlevel is prevented from increasing (e.g., either decreased ormaintained). If the ablation is complete, the medical system stopsdelivering energy.

During a particular medical method, a power source may be coupled to amedical device having at least one ablation element such as for examplean electrode. The ablation element of the medical device may be advancedinto contact with tissue to be treated. The physician may then observevarious parameters, including for example a cardiac pulse waveform,individual or aggregate electrical signals from the ablation elements,confirming positioning of the distal treatment assembly, and settingvarious parameters on the power source. A power level of energy may beselected to ablate the tissue. Upon activation by the physician, thepower source begins delivering energy at the selected power level to theablation element or elements. During activation, the electroniccircuitry and/or processor of the power source monitors the feedbackinformation provided by the medical device, which may includetemperature information. Based on the feedback information, the systemmay determine whether an ablation element is in continuous contact, orhas lost contact, or is in intermittent contact with the tissue. Thatis, the power delivery as set by the duty cycle is limited by algorithmwhen the power delivery is associated with a targeted temperature. Whenan ablation element ceases to be in continuous contact with the tissue,the system may respond appropriately, which may include reducing ormaintaining the power level of the power source.

One possible indicator of lost or intermittent contact is that thetemperature of an ablation element may drop, as compared to an averageof recent temperatures. In other words, when the current orinstantaneous temperature diverges from the average temperature, it mayindicate the ablation element is no longer in continuous contact withthe tissue. A medical method may include advancing an electrode of amedical device into contact with tissue to be treated, monitoring atemperature of the electrode by measuring instantaneous temperature, andcalculating an average temperature of the electrode. A desiredtemperature and a threshold variation may be selected, and energydelivered to the electrode. The method may further include calculating adifference between the instantaneous temperature and the averagetemperature, and a delta value may be calculated by subtracting thedifference from the desired temperature. Reducing the selected powerlevel may be performed by reducing the desired temperature. The desiredtemperature may be reduced when the continuity value exceeds thethreshold variation. In addition, a medical method may also includeselecting an increment (e.g., a pre-determined temperature or poweramount or factor) by which the power may be reduced. A reduction valuemay be calculated by multiplying the continuity value and the increment.Then the desired temperature may be reduced specifically by subtractingthe reduction value from the desired temperature.

After an indication that the ablation element is not in continuouscontact with the tissue, and a corresponding reduction of power level ofenergy from the power source, the medical system may continue to gatherfeedback information and evaluate whether the ablation element hasregained continuous contact with the tissue. Upon determining that theablation element has regained continuous contact, the medical device mayrespond appropriately, including increasing the selected power level ofthe power source. Accordingly, when the continuity value falls below thethreshold variation, indicating the ablation element is again incontinuous contact with the tissue, the desired temperature maythereafter be increased.

A particular example is illustrated in FIG. 10, relating to RF ablationof cardiac tissue. A threshold variation may be selected which is smallenough to provide a rapid response to non-continuous contact, yet avoidunnecessarily frequent adjustments. An increment may be selected whichis small enough to provide responsive control of the ablation process,yet which is large enough to adjust the power level if the ablationelement has lost continuous contact. Any suitable threshold variationand increment may be selected. In the particular example of FIG. 10, thethreshold variation has been selected as approximately 5 degrees, andthe increment is approximately 1 degree. An example graph is shown inFIG. 11, in which the instantaneous temperature oscillates, the averagetemperature rises to maintain a relatively steady temperature ofapproximately 50 degrees, and an initial temperature goal has been setby an operator at 60 degrees. When the instantaneous temperature exceedsthe average temperature by more than the threshold variation, the systemconcludes the ablation element has intermittent contact andautomatically lowers the target temperature. Accordingly, peaks ofcurrent temperature may be controlled to magnitudes within the thresholdvariation.

A decay or delay interval may also be selected, such that a subsequentincrease in power level is performed only after the ablation element hascontinuous contact with the tissue for a time at least equal to thedelay interval. In some situations if the instantaneous temperatureexperiences large or consistent fluctuation, it may momentarily dipbelow the threshold variation, yet soon or immediately rise above thethreshold variation again. Using a delay interval may have the benefitof waiting a designated time period before allowing the temperature goalto reset, resulting in a more stable power control system. As aparticular example, the delay interval may be selected at approximately3 seconds.

In a medical device having a plurality of ablation elements, theprocesses of delivering energy to and monitoring feedback from theablation element, determining whether the ablation element has ceased tobe in continuous contact with the tissue, and reducing the power levelof the power source may be performed individually with respect to eachablation element.

FIGS. 12 and 13 show comparative graphs of another specific examplesystem, in which FIG. 12 illustrates temperature feedback havingrelatively large and consistent oscillation, using a power controlalgorithm to respond accordingly. This response is indicated by thecorresponding oscillating power curve, so as to maintain maximumtemperatures within a threshold variation of a temperature goal. In thisparticular example, the threshold variation has been selected at 5degrees, and the temperature goal at 60 degrees. In a contrastingexample of temperature feedback having relatively little oscillation asshown in FIG. 13, the same power control algorithm applies more subtlecontrol to maintain the desired temperatures and optimize ablationperformance.

With reference to FIGS. 14-18, a medical method may also includeadvancing an electrode of a medical device into contact with tissue tobe treated. A desired temperature and a threshold temperature value maybe selected. Energy is delivered from a power source at a duty cyclevalue to the electrode. Power produced by the power source may bemonitored, as well as a temperature of the electrode. A duty cycle limitmay be set equal to an initial duty cycle value, and energy is limitedto a maximum duty cycle limit when the measured temperature exceeds thethreshold temperature.

With reference to FIG. 14 and a particular example of an RF ablationgenerator with power output modulated by duty cycle, a sensor may beprovided such as a temperature sensor which is near or touching anablation element or an interface between the ablation element and thetissue. A desired temperature may be selected, ablation may begin, and atemperature of the ablation element may be monitored. If the temperatureis not at least equal to the desired temperature, the current duty cyclemay be stored in memory as a maximum limit of the duty cycle. If thetemperature exceeds the temperature threshold, then it may be concludedthe ablation element has contacted the tissue to be treated, and thecontrol system may allow the duty cycle to adjust above the current dutycycle limit. However, if the temperature is below the temperaturethreshold, then it may be concluded the ablation element is not incontact with the tissue to be treated, and the control system maycontinue to limit the duty cycle to, at most, the current duty cyclelimit. Operation according to this particular example is illustrated inFIG. 15, showing a duty cycle being limited in the horizontal portionsof the duty cycle graph.

Specific examples of data recorded during operation of an example systemare shown in FIGS. 16-18. In particular, FIG. 16 illustrates a scenarioin which an ablation element has continuous contact with the selectedtissue for approximately 20 seconds, followed by no contact with thetissue. The temperature initially ascends above a previously selectedthreshold temperature of 50 degrees and maintains at approximately adesired temperature of 60 degrees. Of course, the threshold temperatureand desired temperature may be selected to have any suitable orpreferred magnitudes. After 20 seconds, the temperature descends tobelow 40 degrees, and the algorithm recognizes that the electrode orablation element has lost continuous contact with the tissue. Thealgorithm accordingly limits the duty cycle of the power sourcethereafter.

Another specific example is shown in FIG. 17, depicting a scenario inwhich an ablation element has continuous contact with the selectedtissue for the duration, with a momentary adjustment in contact after 20seconds. The temperature initially rises above a previously selectedthreshold temperature of 50 degrees and maintains at approximately thedesired temperature of 60 degrees. After 20 seconds, the temperaturefalls slightly but remains above the previously selected thresholdtemperature of 50 degrees. The algorithm recognizes that the ablationelement is not in continuous contact with the tissue. The algorithmaccordingly allows the duty cycle to increase and optimize the ablation.

FIG. 18 shows a specific example similar to FIG. 17, in which anablation element has continuous contact with the selected tissue for theduration, with a momentary adjustment in contact after 20 seconds.However, after 20 seconds the temperature falls below the selectedthreshold temperature of 50 degrees. The algorithm accordingly limitsthe duty cycle until the temperature again rises above the selectedthreshold temperature, thereafter allowing the duty cycle to increase.

Alternatively, a medical method may also be provided in which a maximumpower amount is used to limit the average power, as well as theinstantaneous power. An electrode may be advanced into contact withtissue to be treated, and a desired maximum power selected. Energy maybe delivered from a power source at a duty cycle value to the electrode.Power produced by the power source is monitored, and average powerproduced by the power source is calculated. The average power is limitedto the desired maximum power. When the instantaneous power reaches thedesired maximum power, the corresponding duty cycle is stored as amaximum duty cycle. Thereafter, the duty cycle value is limited to themaximum duty cycle.

Of note, although the methods described herein involve targettemperatures and duty cycle modifications and control logic to providethe desired treatment and power delivery characteristics, it is alsocontemplated that voltage control modalities may be implemented withcontinuous wave radiofrequency ablation devices. For example, in deviceswhere radiofrequency power delivery is substantially constant (e.g.,does not include off-periods of a duty cycle), the voltage of thedelivered power or signal can be set and tailored depending on themeasured and desired temperatures, akin to that described herein withrespect to duty cycle modifications.

It will be appreciated by persons skilled in the art that the presentinvention is not limited to what has been particularly shown anddescribed herein above. Of note, while certain components, such as thevarious electrodes or other items disclosed herein, are indicated asmapping, reference, and/or recording electrodes, it is understood theseare exemplary functions that do not limit additional uses of thedesignated electrodes or components for alternative functions. Inaddition, unless mention was made above to the contrary, it should benoted that all of the accompanying drawings are not to scale. A varietyof modifications and variations are possible in light of the aboveteachings without departing from the scope and spirit of the invention,which is limited only by the following claims.

What is claimed is:
 1. A medical method, comprising: advancing anablation element of a medical device into contact with tissue to betreated; supplying power from a power source to the ablation element toablate the tissue; monitoring a temperature of the ablation element;then determining a state of contact between the ablation element and thetissue based on the temperature of the ablation element; then, after adetermination that the state of contact is that continuous tissuecontact has been lost, initiating a reduction in the power supplied bythe power source; and when the ablation element regains continuoustissue contact; and the state of regained continuous tissue contact hascontinued for at least three seconds, then initiating an increase in thepower supplied by the power source.
 2. The medical method of claim 1,wherein the tissue to be treated is cardiac tissue.
 3. The medicalmethod of claim 1, further comprising: selecting a desired temperatureof the ablation element; and reducing the power to the ablation elementwhen the monitored temperature exceeds the desired temperature.
 4. Themedical method of claim 1, further comprising: selecting a maximum limitfor the supplied power to the ablation element; measuring the suppliedpower to the ablation element; calculating the average power supplied tothe ablation element; and limiting the average power to the maximumthreshold.
 5. The medical method of claim 4, further comprising settinga maximum duty cycle equal to the current duty cycle value when thesupplied power is at least equal to the maximum limit, and thereafterlimiting the duty cycle value to the maximum duty cycle.
 6. A medicalmethod, comprising: advancing an electrode of a medical device intocontact with tissue to be treated; delivering energy from an energysource to the electrode; monitoring an instantaneous temperature of theelectrode; selecting a desired temperature and a threshold variation;calculating an average temperature of the electrode during the energydelivery to the electrode; calculating a difference between theinstantaneous electrode temperature and the average electrodetemperature; calculating a continuity value by subtracting thedifference from the desired temperature; determining a state of contactbetween the electrode and the tissue and assigning a contact status, thecontact status based on the continuity value and the thresholdvariation, a contact status of continuous contact being assigned whenthe continuity value is less than the threshold variation and a contactstatus of one of intermittent contact and no contact being assigned whenthe continuity value is greater than the threshold variation; initiatinga reduction in the energy delivered by the energy source when thecontact status is one of intermittent contact and no contact; thenre-evaluating contact between the electrode and the tissue andre-assigning the contact status; determining whether the contact statusis continuous contact; and when the contact status is continuous contactand has been continuous contact for at least three seconds, theninitiating an increase in the energy delivered by the energy source. 7.The medical method of claim 6, wherein the threshold variation issubstantially equal to 5 degrees.
 8. The medical method of claim 6,further comprising selecting an increment, and calculating a reductionvalue by multiplying the continuity value by the increment, whereinreducing the desired temperature is performed by subtracting thereduction value from the desired temperature.
 9. The medical method ofclaim 8, wherein the increment is substantially equal to 1 degree. 10.The medical method of claim 7, wherein the medical device has aplurality of electrodes, and wherein monitoring, delivering,calculating, and reducing are performed individually with respect toeach electrode.
 11. A medical method, comprising: advancing an electrodeof a medical device into contact with tissue to be treated; monitoring atemperature of the electrode; selecting a desired temperature and athreshold value; delivering energy from an energy source at a duty cyclevalue to the electrode; setting a duty cycle limit equal to an initialduty cycle value; limiting the energy to a maximum of the duty cyclelimit when the threshold value exceeds the temperature; and after thetemperature has exceeded the threshold value continuously for at leastthree seconds, then increasing the duty cycle limit.
 12. The medicalmethod of claim 11, wherein the threshold value is substantially equalto 50 degrees.